1. Field of the Invention
In distributed industrial automation systems, when measurement and control data are captured, evaluated and transmitted, it is necessary to ensure that complete and unaltered data are available in real time, particularly in the case of time critical an industrial production processes. Typically, industrial automation system comprises a multiplicity of automation devices that are networked to one another by an industrial communication network, and is used within the context of production or process automation to control or regulate installations, machines or devices. Reports that are not transmitted or that are not transmitted completely can prevent an industrial automation system from changing to or remaining in a safe operating state, for example. This can finally result in failure of an entire production installation and in a costly production stoppage. A particular set of problems regularly arises in industrial automation systems from report traffic with comparatively many but relatively short messages, which amplifies the above problems.
2. Description of the Related Art
In order to be able to compensate for failures in communication links or devices, communication protocols, such as media redundancy protocol, high-availability seamless redundancy or parallel redundancy protocol, have been developed for high-availability, redundantly operable in industrial communication networks. Media redundancy protocol (MSR) is defined in the International Electrotechnical Commission (IEC) 62439 standard and allows compensation for individual connection failures in networks with a simple ring topology in the event of bursty redundant transmission of messages.
Based on media redundancy protocol, a switch having two ports within the ring topology has an associated redundancy manager that monitors the network for connection failures and initiates a switching measure for ring closure if necessary. In the normal operating state, the redundancy manager uses test messages to check whether an interruption has occurred within the ring topology. However, the switch associated with the redundancy manager does not normally forward messages containing useful data from one port to the other port. This prevents messages that contain useful data from circulating constantly within the ring topology. If a switch or a connection fails within the ring topology, test messages emitted by a port are no longer received at the respective other port. From this, the redundancy manager can recognize a failure and, in the event of a failure, the redundancy manager forwards messages containing useful data from one port to the other port and vice versa, unlike in the normal operating state. Furthermore, the redundancy manager prompts the remainder of the switches to be notified about a failure-dependent topology change. This prevents messages from being transmitted via the failed connection.
Bursty media redundancy methods can be implemented with relatively little complexity in principle. However, it is disadvantageous that firstly messages can be lost in the case of error and secondly there is initially a fault state during reconfiguration of a communication network. Such a fault state needs to be backed up by a superimposed communication protocol, for example, by Transmission Control Protocol/Internet Protocol (TCP/IP) at network or transport layer level, in order to prevent an interruption in a communication link.
PROFINET (IEC 61158 Type 10) also refers to media redundancy protocol as a bursty media redundancy method within a communication network with a ring topology. By contrast, media redundancy planned duplication (MRPD) provides an extension for smooth transmission of isochronous realtime data. Media redundancy planned duplication is not an application-neutral smooth media redundancy method, however, but rather a PROFINET-specific extension.
High-availability seamless redundancy (HSR) and parallel redundancy protocol (PRP) are defined in the IEC 62439-3 standard and allow smooth redundant transmission of messages with extremely short recovery times. Based on high-availability seamless redundancy and parallel redundancy protocol, each message is duplicated by a sending communication device and is sent to a receiver in two different ways. A receiver-end communication device filters out redundant messages that are duplicates from a received data stream.
EP 2 282 452 A1 describes a method for data transmission within a ring-like communication network, in which the data transmission occurs based on high-availability seamless redundancy and the communication network comprises at least a master node, a source node and a destination node. Each node has a first and a second communication interface with a respective first and second neighboring node. Furthermore, each node receives data frames via the first communication interface and forwards the received data frame in either altered or unaltered form via the second communication interface without additional delay. The master node sends a first and a second redundant data frame or an empty data frame to its first or second neighboring node. When the two redundant data frames are received, the source node fills the respective data frame in a predetermined reserved area with process data. Next, each filled data frame is immediately and individually forwarded to the first or second neighboring node of the source node. Finally, the destination node extracts the process data from the first received filled data frame in a pair of redundant data frames.
EP 2 413 538 A1 discloses a method for redundant communication in a communication system that comprises a plurality of communication networks. The communication networks are connected to one another by at least one coupling node. Transmission of data that come from a first communication network back to the first communication network from a second communication network is prevented based on a segment of information that is defined prior to data transmission.
The IEC 62439-3 standard prescribes hitherto exclusively wired transmission links for the parallel redundancy protocol (PRP) based on comparatively long latency delays in wireless communication systems and a resultant nondeterministic transmission response. “Towards a Reliable Parallel Redundant WLAN Black Channel”, Markus Rentschler, Per Laukemann, Institute of Electrical and Electronic Engineers (IEEE) 2012, examines suitability of WLAN transmission links in PRP communication networks. Parallel application of various diversity techniques for space, time and frequency, for example, can be used to compensate adequately for effects of stochastic channel fading in WLAN communication networks.